Networks enable computer users to take advantage of their organization's total computing capabilities through information exchange and sharing of resources. Typically, computer resources are arranged into local area networks (LANs) when high speed data transfer is needed within a building or cluster of buildings.
However, the widely-used standard Ethernet LAN has a distance limitation of 2,800 meters between the farthest two nodes. This distance can be extended to several miles by using a device called a LAN bridge to form a so-called extended LAN. A bridge uses appropriate media such as leased telephone lines, fiber optic cables, or microwave repeaters to interconnect two LANs. Bridges are also used to isolate traffic on one section of a LAN from that on another section, and thus bridges also afford a way to improve the performance of a LAN.
Data transmission over longer distances can be accomplished by providing access to remote sites, or stations, through dedicated modems and leased telephone lines. An organization having offices in geographically remote cities, but desiring to have the same interconnection flexibility available in a LAN, is likely to configure its computers into a so-called wide area network (WAN). WANs employ a number of different long-distance communications media such as leased telephone lines, public data networks, and microwave radio or satellite links.
In large organizations, it is now quite common to have a variety of LANs, extended LANs, and WANs. The ideal arrangement for such an organization is to have the user see all of organization's networks function as a single entity, regardless of whether the networks consist of LANs, extended LANs, or WANs, and independently of how these networks are interconnected.
Clearly, a single wide-area network can have a large number of devices on it, the quantity and types of which can vary with time. To operate properly, communications between devices at different locations must include in the messages some indication of the location and identity of the device for which the message is intended. Furthermore, each location must have the capability of determining whether it is the destination designated by the message. Additionally, since the composition of the network can change, devices at the various locations have to be kept apprised of the locations and identities of all or many of the other devices on the network.
Providing all these "housekeeping functions" can exact a significant bandwidth penalty, and it is of course important to keep the percentage of bandwidth dedicated to transmission of actual data, as opposed to housekeeping information, as high as possible. It is also desirable to keep the percentage of hardware costs dedicated to interface equipment as opposed to actual terminal equipment to be as low as possible, preferably by standardization. However, the protocol sometimes requires that certain station- and/or device-specific circuitry be provided so that a device can distinguish messages intended for it from messages intended for other devices on the wide-area network.